[systemd-devel] Creating executable device nodes in /dev?

[Andy Lutomirski]

On Thu, Nov 19, 2020 at 10:05 AM Topi Miettinen <toiwoton at gmail.com> wrote:
>
> On 19.11.2020 18.32, Zbigniew Jędrzejewski-Szmek wrote:
> > On Thu, Nov 19, 2020 at 08:17:08AM -0800, Andy Lutomirski wrote:
> >> Hi udev people-
> >>
> >> The upcoming Linux SGX driver has a device node /dev/sgx.  User code
> >> opens it, does various setup things, mmaps it, and needs to be able to
> >> create PROT_EXEC mappings.  This gets quite awkward if /dev is mounted
> >> noexec.
> >>
> >> Can udev arrange to make a device node executable on distros that make
> >> /dev noexec?  This could be done by bind-mounting from an exec tmpfs.
> >> Alternatively, the kernel could probably learn to ignore noexec on
> >> /dev/sgx, but that seems a little bit evil.
> >
> > I'd be inclined to simply drop noexec from /dev by default.
> > We don't do noexec on either /tmp or /dev/shm (because that causes immediate
> > problems with stuff like Java and cffi). And if you have those two at your
> > disposal anyway, having noexec on /dev doesn't seem important.
>
> I'd propose to not enable exec globally, but if a service needs SGX, it
> could use something like MountOptions=/dev:exec only in those cases
> where it's needed. That way it's possible to disallow writable and
> executable file systems for most services (which typically don't need
> /tmp or /dev/shm either). Of course the opposite
> (MountOptions=/dev:noexec) would be also possible, but I'd expect that
> this would be needed to be used more often.
>

I imagine the opposite would be more sensible.  It seems odd to me
that we would want any SGX-using service to require both special mount
options and regular ACL permissions.

As  a further argument, I just did this on a Fedora system:

$ find /dev -perm /ugo+x -a \! -type d -a \! -type l

No results.  So making /dev noexec doesn't seem to have any benefit.